Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks (MOFs)

Summary

Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Abstract

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of the reaction site in metal-organic framework (MOF)–based catalysts in an enantioselective catalytic reaction system. Therefore, a method of validating the reaction site of MOF-based catalysts is necessary to investigate this issue. Substrate size discrimination by pore size was accomplished by comparing the substrate size versus the reaction rate in two different types of carbonyl-ene reactions with two kinds of MOFs. The MOF catalysts were used to compare the performance of the two reaction types (Zn-mediated stoichiometric and Ti-catalyzed carbonyl-ene reactions) in two different media. Using the proposed method, it was observed that the entire MOF crystal participated in the reaction, and the interior of the crystal pore played an important role in exerting chiral control when the reaction was stoichiometric. Homogeneity of the chiral environment of MOF catalysts was established by the size control method for a particle used in the Zn-mediated stoichiometric reaction system. The protocol proposed for the catalytic reaction revealed that the reaction mainly occurred on the catalyst surface regardless of the substrate size, which reveals the actual reaction sites in MOF-based heterogeneous catalysts. This method for reaction site validation of MOF catalysts suggests various considerations for developing heterogeneous enantioselective MOF catalysts.

Introduction

MOFs are considered a useful heterogeneous catalyst for chemical reactions. There are many different reported uses of MOFs for enantioselective catalysis^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,}....

Protocol

1. Preparation of (S)-KUMOF-1 crystals in three sizes

NOTE: Each step follows the experimental section and supplementary information of previous reports^2,24,27. Three different sizes of (S)-KUMOF-1 were prepared: large (S)-KUMOF-1-(L), medium (S)-KUMOF-1-(M), and small (S)-KUMOF-1-(S) with particle sizes >100 μm, >20 μm, and <1 μm, respectively. When out of the solvent, (S)-KUMOF-1 disma....

Results

The enantioselective carbonyl-ene reaction using the Zn reagent is stoichiometric because of the difference in the binding affinities of the alkoxy and carbonyl groups to the metal (Figure 2). For this reason, the substrates were converted into the products at the reaction site and remained there. The desired products were obtained by dismantling the crystals, as detailed in section 4 of the protocol. The results of the heterogeneous enantioselective carbonyl-ene reaction of substrates by Zn.......

Discussion

After the synthesis of (S)-KUMOF-1, crystals in some vials seem to be powdery and are not appropriate for use in catalysis. Therefore, proper crystals of (S)-KUMOF-1 need to be selected. The yield of (S)-KUMOF-1 is calculated using only those vials in which it was successfully synthesized. When withdrawn from the solvent, (S)-KUMOF-1 dismantles. Therefore, the crystals should always be kept wet. For this reason, weighi.......

Materials

Name	Company	Catalog Number	Comments
Acetone	Daejung	1009-4110
Analytical Balance	Sartorius	CP224S
Copper(II) nitrate trihydrate	Sigma Aldrich	61194
Dichloromethane	Daejung	3030-4465
Dimethyl zinc	Acros	377241000
Ethyl acetate	Daejung	4016-4410
Filter paper	Whatman	WF1-0900
Methanol	Daejung	5558-4410
Microwave synthesizer	CEM		Discover SP
Microwave synthesizer 10 mL Vessel Accessory Kit	CEM	909050
N,N-Diethylformamide	TCI	D0506
N,N-Dimethylaniline	TCI	D0665
n-Hexane	Daejung	4081-4410
Normject All plastic syringe 5 mL luer tip 100/pk	Normject	A5
Pasteur Pipette 150 mm	Hilgenberg	HG.3150101
PTFE tape	KDY	TP-75
Rotary Evaporator	Eyela	243239
Shaker	DAIHAN Scientific	DH.WSO04010
Silica gel 60 (230-400 mesh)	Merck	109385
Synthetic Oven	Eyela	NDO-600ND
Titanium isopropoxide	Sigma Aldrich	87560
Vial (20 mL)	SamooKurex	SCV2660
Vial (5 mL)	SamooKurex	SCV1545